Summary of work: To understand the precursors of adult personality development, longitudinal, cross-sectional, and cross-cultural analyses were conducted on adolescents aged 12 to 18. Participants completed the adult version of a measure of the Five-Factor Model, which has recently been shown to be valid when used in this age group. In a sample of gifted students retested after four years, the only consistent change was an increase in Openness to Experience. This finding was replicated in cross-sectional analyses of responses from unselected adolescents in the US and Belgium. Although individuals change in the levels of personality traits in this portion of the lifespan, there are few consistent developmental trends. Major changes occur during the period from 18 to 30. In the U.S., Neuroticism, Extraversion, and Openness decrease between late adolescence and adulthood, whereas Agreeableness and Conscientiousness increase. Those findings were replicated cross-culturally in a sample of 36 cultures from five continents. Such pan-cultural trends suggest that personality change is an intrinsic aspect of the aging process. Both cross-cultural and longitudinal studies on personality, stress, and coping will continue.